Final Report

Aayush Bhardwaj| Roll No. – 06438, E&CED

SUMMER INTERNSHIP PROGRAM FINAL REPORT

Name of the Company: Idea Cellular Limited,

(Formerly Spice Telecom Ltd.)

Mohali

GSM Overview and NSS Operation

A REPORT ON


BY

AAYUSH BHARDWAJ

Roll No. 06438

CLASS OF 2010

DEPARTMENT OF ELECTRONICS & COMMUNICATION ENGINEERING

A Report Submitted In Partial Fulfillment of the Requirements of

B. Tech Program Of

NIT Hamirpur


ACKNOWLEDGEMENTS

As part of the B.Tech. curriculum at NIT Hamirpur, the ‗Summer Internship Program‘ aims

at overall development of the students by providing them an opportunity to gain corporate

exposure and space to apply their theoretical knowledge in practice in a mutually beneficial

manner. No summer internship can be successful without the support of the people who

keep themselves closely involved with the student undergoing the program. The wealth of

knowledge and guidance shared and provided to us by these professionals is invaluable.

My sincere regards to Mr. Taranjit Singh, Asstt. Manager – HR, Idea Cellular Ltd., Mohali,

and to Mr. Saurabh Prashar, Sr. Manager (Technical), Idea Cellular Ltd., Mohali, for allowing

me to pursue this project. I would also like to thank my project company guide at Idea Cellular

Ltd, Mohali, Mr. Gurdeep Singh, for his selfless support and encouragement during my entire

training program.

I would also like to extend my gratefulness to all the employees of Idea Cellular Ltd, Mohali,

who with their patience and co-operation, have made my internship a rewarding and

meaningful experience. I am grateful for each and every valuable interaction that brought me to a

better understanding of the finer aspects which were instrumental in helping me carry out the

project in effective manner.

Aayush Bhardwaj


LIST OF DIAGRAMS

Title of Diagram Page No.

Figure 1: Increasing Subscriber Base in the Telecom Market in India………….…….……........

Figure 2: Operator Wise Market Share of GSM Service Providers (Dec‘08)…….…….……….

Figure 3: Operation circles of Idea Cellular………………………………….…………….…….

Figure 4: Table showing GSM frequency bands…………………………….……….……….….

Figure 5: GSM Cell system...………………………………..………………...…………………

Figure 6: Basic cell shape…………………………………………………………...……………

Figure 7: Cell sectorization……………………………………………………...….……………

Figure 8: Map depicting cell planning……………………………………………..…………….

Figure 9: The 4/12 cell cluster pattern…………………………………………..……………….

Figure 10: The 3/9 cell cluster pattern…………………………………………..……………….

Figure 11: GSM evolution………………………………………………………..………………

Figure 12: GSM Network components…………………………………………….…………….

Figure 13: Mobile Equipment…………………………………...………………….……………

Figure 14: SIM card……………………………………………...…….………….………..……

Figure 15: BSS Components………………………………………………….………………….

Figure 16: NSS Architecture…………………………………………………….…………….…

Figure 17: MS Registration process………………………………………………….…………..

Figure 18: IMEI check procedure…………………………………………….……….…………

Figure 19: GSM Call procedure…………………………………………………….……………

Figure 20: MOC initiation and completion………………………………………….…………...

Figure 21: MTC initiation and completion………………………………………………………

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TABLE OF CONTENTS Title Page No.

Introduction……………………………………..……………………………….

The Aditya Birla Group……………….………………………………………….

Idea Cellular Limited…………………….…..…………………………………..

Idea Cellular Limited, Mohali…………………………………………………..

Global System for Mobile Communication (GSM)…………………………….

Frequency Spectrum…………………………………………………………….

GSM Cells……………….…………….…………….……………..…………….

o Cell size and shape……………….…………………………………………….……………………………. o Cell coverage……………….…………………………………………………….………….…………………. o Cell planning……………….…………………………………………………….………….…………………. o Cell clusters……………….…………………………………………………….………………………………. o Switching and control……………….………………………………………………..…………………….

Features of GSM……………….…………………………….………………….

GSM generations……………….……………………………………………….

GSM network components……………….………………….………………….

Mobile Station (MS) ……………….……………………………………………. o Mobile Equipment (ME) ……………….…………………………………………………………………. o Subscriber Identity Module (SIM) ……………….……………………………..…………………….

Base Station System (BSS) ……………….…………………………………… o Base Station Controller (BSC) ……………….…………………………………………………………. o Base Transceiver Station (BTS) ……………….……………………………………………………….. o Transcoder (XCDR) ……………….………………………………………………………………………….

Network Switching Subsystem (NSS) ……………….………..………………. o Mobile Services Switching Center (MSC) …………………………………………………………. o Home Location Register (HLR) ……………….…………………………..……………………………. o Visitor Location Register (VLR) ……………….………………………………………………….……. o Equipment Identity Register (EIR) ……………….…………………..………………………………. o Authentication Center (AUC) ……………….………………………………………………….………. o Interworking Location Register (ILR) ……………….………………………………………………. o Echo Canceller (EC) ……………….…………………………………..……………………………………. o Data Transfer Interface (DTI) ……………….………………………….………………………………. o Message Center (MC) ……………….………………….………………………………………………….

7 9 10 12 15

16

18 18 20 20 22 23

24 27 29 30 30 30 32 32 33 33

34 35 35 36 37 37 38 38 38 39


Mobile Services Switching Center (MSC) ……………….……………………. o Services……………….…………………………………………………….….…………………………………. o Functions……………….………………………………………………………………………………………… o Switching network……………….………………………………………………………………………….. o Line Trunk Group……………….………………………………………….………………………………….

Registration of MS on network……………….………….……………………. o IMEI check……………….………………………………………………………………………………………. o GSM Authentication process……………….…………………..………………………………………. o Location update……………….………………………………………………..……………………………. o Interrogation……………….…………………………………………………….……….……………………. o Handover……………….……………………………………………………………….….…………………….

GSM Call procedure……………….……………………….……………………. o Mobile Originating Call (MOC) ……………….…………………………………………………….…. o Mobile Terminating Call (MTC) ……………….………………………………………………………. o Mobile to Mobile Call (MMC) ……………….…………………………………………………………. o Call Charge Registration……………….………………………………………………….……………….

Conclusion……………………………………………………………………….

References………………………………………………………………………..

Glossary…………………..……………………………………………………..

41 41 43 43 44 46 46 47 48 49 50

51 52 53 54 54 55 56 57


Introduction

The world has become a global village, and India has become an integral part of this village. In

1993, Dr. Manmohan Singh put in place new policies in the Annual Budget, allowing private

players entry into the telecom segment. Also, a comprehensive policy was put into place for the

wireless telephony segment that was introduced the following year.

Mobile telephony was introduced in Indian markets in 1994. In the last few years, the sector has

witnessed tremendous growth, with more and more players entering it. The service providers are

also adding more and more customers every year. Liberalized policies have ensured lower tariffs

and reduced roaming rentals, leading to increased usage of mobile phones.

At 400 million connections, the ‗Indian Telecom Industry' is the fifth largest and fastest growing

in the world. The subscriber base grew by over 20% in 2008 and is expected to reach 600 million

by 2013. Over the last 3 years, 75% of the new telephone connections were wireless, which now

accounts for 54.6% of the total telephone subscriber base. The wireless technologies currently in

use in India are Global System for Mobile Communications (GSM) and Code Division Multiple

Access (CDMA). 3G too has now been introduced in the market.

Source: http://www.zdnetasia.com/insight/specialreports/india/

Fig. 1: Increasing Subscriber Base in the Telecom Market in India

Mobile telephony provides services such as messaging- text and multimedia- mobile commerce

through GPRS enabled mobile Internet, with local calls and long distance calls- national and

international. Not only service providers but also equipment manufacturers are contributing

towards the growth of the sector. Mobile telephony started up with bulky handsets and has now

reached to smart phones with cameras, radio facility and lots of other multimedia applications.

Also, PDAs have entered Indian markets with operating systems that make it a pocket PC.

http://www.zdnetasia.com/insight/specialreports/india/


Three types of players exists in the Indian Telecom community -

State owned companies like - BSNL and MTNL.

Private companies like - Idea Cellular, Reliance Infocomm and Tata Teleservices.

Foreign invested companies like – Bharti Tele-Ventures, Escotel, BPL Mobile, etc.

Source: http://www.medianama.com/wp-contentuploads2008

Fig. 2: Operator Wise Market Share of GSM Service Providers (Dec‘08)

http://www.medianama.com/wp-contentuploads2008


The Aditya Birla Group

A US $28 billion corporation, the Aditya Birla Group is in the league of Fortune 500. It is

anchored by an extraordinary force of 130,000 employees. Over 50 per cent of its revenues flow

from its overseas operations in 25 countries across Asia, Europe, Africa and the Americas.

Globally the Aditya Birla Group is:

A metals powerhouse.

No.1 in viscose staple fiber.

The fourth largest producer of insulators.

The 11th largest cement producer globally.

Among the worlds top 15 BPO companies and India's fifth largest.

In India:

A premier branded garments player.

The second largest player in viscose filament yarn.

Among the top five mobile telephony companies.

Among the top three supermarket chains in the retail business

History

The roots of the Aditya Birla Group date back to the 19th century in the town of Pilani. Seth Shiv

Narayan Birla started trading in cotton, laying the foundation for the House of Birlas. Through

the 1850s, the Birla business expanded rapidly. In the early 20th century, the group set up

industries in critical sectors such as textiles, fibre, aluminium, cement and chemicals.

Mr. Aditya Birla dared to dream of setting up a global business empire. He ventured to set up

world-class production bases, having foreseen the winds of change and staked the future of his

business on a competitive, free market driven economy order. He put Indian business on the

globe, 22 years before economic liberalization was formally introduced.

Companies

Indian Companies

Grasim – It is one of the leading manufacturers of fibers, cement, textiles etc.

Hindalco – it is one of the oldest entities of the conglomerate, and operates in mining of

aluminium, copper and other metals.

Aditya Birla Nuvo –this is one of the flagship companies, and the main promoter of

Idea Cellular Ltd. , Birla Sun Life Insurance Co. Ltd. , PSI Data Systems etc.

International Companies

The Aditya Birla Group has a significant presence in Europe, South East Asia, Africa, North

America, Australia and China as well. The Group operates in 25 countries. Few of its

international ventures include: Thai Rayon, Indo Thai Synthetics, Indo Phil Group of

Companies, PT Indo Bharat Rayon, A.V Group, Aditya Birla Minerals Limited, Novelis etc.

http://www.adityabirlainsulators.com/

http://www.grasim.com/

http://www.minacs.adityabirla.com/

http://www.adityabirlanuvo.com/maduragarments/index.aspx

http://www.adityabirlanuvo.com/indianrayon/index.aspx

http://www.ideacellular.com/

http://www.morestore.com/


!dea Cellular Limited

Idea Cellular Limited was incorporated in 1995 by the Aditya Birla Group and is one of the

leading GSM mobile services operators. Headquartered in Mumbai, it has licenses to operate in

all 22 service areas across the country, though commercial operations are currently in 16 services

areas. With a customer base of over 43.02 million subscribers, the company's footprint at present

covers around 45% of the population of India and more than 50% of the telecom market.. The

chairman of the company is Mr. Kumar Mangalam Birla and the managing director is Mr.

Sanjeev Aga.

IDEA enjoys a market leadership position in many of its operational areas. It offers GPRS on all

its operating networks, and was the first company in India to commercially launch the next

generation EDGE technology in Delhi in 2003. As a pioneer in technology deployme nt, it has

been in the forefront through the adoption of bio fuels to power its base stations, and by

employing satellite connectivity to reach inaccessible rural areas in Madhya Pradesh.

Idea Cellular Limited was the 1st cellular company in the country to launch the scheme of

music messaging with background tones, cellular jockey, and group talk. The company was also

the 1st to launch mobile email services and voice portal. The company also offers roaming, value

added services, and call management services to its customers.

IDEA has won numerous awards and is the only Indian GSM operator to win the prestigious

GSM Association Award consecutively in the best mobile technology category for the ‗Best

Billing and Customer Care Solution‘ twice in 2006 and in 2007 in the face of international

competition. In 2007 IDEA was listed on the National Stock Exchange (NSE) and the Bombay

Stock Exchange (BSE).

With ambitious future plans, the company is poised for rapid growth.

MISSION STATEMENT: “To be the most customer-focused mobile service brand, continuously innovating to help

liberate our customers from the shackles of time and space.”


Fig. 3: Operation circles of Idea Cellular


!dea Cellular Limited, Mohali

Idea Cellular Limited, Punjab came into existence in after Idea Cellular acquired Spice

Communications Limited in June 2008. While most of the staff was retained, fresh faces were

brought in to consolidate the subscriber base in Punjab circle (which includes Punjab and

Chandigarh) and to improve the services offered.

Idea Cellular has its workforce divided into various departments for streamlined operations and

better efficiency. The departments are:

1. Administrative Department

2. Human Resources Development

3. Corporate Support Group

4. Marketing and Sales Department

5. Technical Department

6. Finance Department

7. Engineering Department

1. ADMINISTRATIVE DEPARTMENT

The administrative department constitutes the core team of the office and works in tandem with

the other departments to ensure effective working. Its functions include:-

a) Cost Control and material distribution.

b) Processing of bills and maintenance of records.

c) Establishment of ROs/Showroom.

d) Security.

e) Transportation.

f) Reservation of Trains, Hotel rooms, etc for touring staff.

g) Salary management of contracted staff.

h) Optimized Utilization of resources.

2. HUMAN RESOURCE DEVELOPMENT (HRD):- This department deals with the employees of the organization, as well as the trainees. HRD is

also involved in providing training to all the employees of the organization. New employees

often need training as a means of their progress in their jobs and career. The requests for

vocational training are also dealt with by this department.

The major functions of the department are:

a) The foremost function of the HRD is defining the manpower requirement, in terms of

skill set desired in the new employees, as well as the number of people to be hired.

Recruitments are carried out in coordination with the other departments.


b) HRD conducts orientation programs involving introduction of the new employees to

organization. Training programs are conducted for both old as well as new employees.

c) HRD is involved in decisions regarding the salary and other benefits, as well as the

increments to be given to the employees, and their distribution.

d) It maintains the daily attendance register of all the employees.

e) When an employee is leaving, an analysis is done by the HRD as to the reasons why the

employee is leaving, so that the steps can be taken to retain the other employees.

3. CORPORATE SUPPORT GROUP

Corporate support group looks after the needs of other departments. It consists of four sections:

a. Site Acquisition, responsible for hiring of buildings sites and construction of antennas

and sites, before handing it over to the Engineering Department. It also looks after the

electrical section of the office.

b. Purchase section looks after the administrative needs of the office in terms of procurement

of stationery, and purchase, repairs and replacement of furniture and other items. Also, hiring

of vehicles, hotel and railway reservations, etc for the officials; as well as maintenance of the

company‘s vehicles and property is done by this section.

c. Warehouse & HO Logistics: Warehouse is the place where all the extra equipment's, SIM

cards, switch cards, etc are kept, till the time they are required. Warehouse & HO Logistics

work in co-ordination with each other. HO Logistics estimates the requirements of various

equipments and places the order for it. It is the responsibility of the warehouse to make

arrangements for receiving the goods and storing them. All customs and excise duties are

paid by this department.

d. Security: The services of Group4 Securities (P) Ltd. have been hired to meet the security

needs of the Mohali Office. All employees are issued ID Cards, while passes are issued to

visitors to the office upon entry in the gate register. The record entry of all vehicles and

material entering and leaving the premises is also done.

4. MARKETING & SALES DEPARTMENT

This department oversees the marketing and sales areas and is responsible for introductio n and

promotion of new schemes, apart from maintenance of the customer assistance centers and

grievance redressal. It also keeps records regarding the number of new connections issued, the

old ones terminated and the like.

5. TECHNICAL DEPARTMENT

It looks after all the technical know-how to be implemented in the office and also facilitates

technical training of the employees. It is responsible for maintenance of the office LAN system,

as well as the WAN connections to the other offices. It updates the intranet periodically, and


performs basic maintenance of the office machinery. Also, changes in the bills of post-paid

customers are implemented as per the schemes opted for by them.

6. FINANCE DEPARTMENT

This department consists of two sections i.e. Accounts & Net Revenue Management Group

(NRMG). This section basically deals with the funds and their utilization. It is also involved with

the preparation of the cheques / drafts for payment to various vendors or to customers (as refund

cheques) who have disconnected the post-paid service.

7. ENGINEERING DEPARTMENT

The engineering department provides round the clock Network availability to all the customers.

This department has six sections:

a) NSS (Network Switching Centre)

It maintains the Switch that routes all the calls through the network.

b) BSS (Base Station System)

It is involved in the maintenance of the GSM Network and involves preventive

maintenance, weather proofing, site expenditures, fuel and power availability, etc. Any

reconfiguration, if required is also done.

c) OMC-R (Operations and Maintenance for Radio)

It handles maintenance and operations of the network. Any alarms in the Network are

raised in OMCR, notification for which are then are sent to the concerned regions.

d) EFD (Engineering Front Desk)

This is the front desk for the department. Any customer complaints or queries from any

other department go through this and are forwarded to the relevant section.

e) Planning

it is involved in the planning regarding the location of antenna sites, testing etc.

f) Performance

This section keeps a check on the Network performance and enhances the performance

whenever required.


Global System for Mobile

Communications (GSM)

History of GSM Due to the increasing use of radio communications throughout Europe, the frequency spectrum

was becoming congested and cluttered. At the World Administrative Radio Conference (WARC)

of 1979, the frequency band to be used was agreed upon, that would replace the different

frequency bands that were being used in different countries throughout Europe.

In 1982, the Conference of European Posts and Telecommunications Administrations (CEPT)

established a committee called “Groupe Speciale Mobile” (GSM) to specify a European

telecommunications standard in the 900 MHz frequency band. This committee was set up to

specify a unique radio communication system for Europe based on the digital format to replace

the analog systems in use at that time.

In 1988 the European Telecommunications Standard Institute (ETSI) was created. This institute

took over most of the activities of CEPT including GSM. This enabled network providers and

telecommunications equipment manufacturers to become involved in the specification of GSM.

In January 1991 phase 1 issue of DCS 1800 (now called GSM 1800) was approved by ETSI.

Commercial services started in 1992. Also during this year, the GSM System was renamed.

Rather than being called ―Groupe Speciale Mobile‖ it was now named ―Global System for

Mobile Communications‖.

The next phase of GSM, 2G, was implemented in 1994, and gained popularity soon due to the

features offered by it. The next phases were 2.5G and 2.75G, which were basically extensions of

the 2G services, and were introduced in 1999 and 2002 respectively.

The latest innovation has been the 3G technology, which offers even faster data transfer rates and

better call facilities, while 4G is expected to be introduced for public usage by 2010.


Frequency Spectrum

The cellular frequencies lie within the UHF band. The frequency spectrum is very congested due

to the narrow bandwidth allotted for cellular communications. The major frequency bands are

known as GSM 400, GSM 900, GSM 1800 (DCS1800) and GSM 1900(PCS 1900).

The capacity of these bands is defined in terms of Absolute Radio Frequency Channel Number

(ARFCN). A single Absolute Radio Frequency Channel Number (ARFCN) is actually a pair of

frequencies, one used in each direction (transmit and receive). This allows information to be

passed simultaneously in both directions. The frequencies are separated by a certain band to

avoid interference of uplink and downlink signals. For each cell in a GSM network at least one

ARFCN must be allocated, and more may be allocated to provide greater capacity.

The GSM 400 bands were the earliest used bands in the Nordic countries and were first

generation systems, but are now obsolete with the adoption of the GSM 900 and GSM 1800

bands. The GSM 850 band is called as Cellular Band, for it was the first band to be allocated for

cellular phones in the US.

The GSM 900 band originally contained 124 ARFCNs, but was later modified to add 50

channels, resulting in the Extended GSM (E-GSM) 900 band, which has 174 channels. GSM

1800 is also called DCS 1800, for Digital Cellular System. Again, GSM 1900 is referred to as

Personal Communications Service, or PCS 1900.

Similarly, a new band, the GSM-Railway (GSM-R) was finalized in 2000 to provide a secure

platform for voice and data communication between railway operational staff, including drivers,

dispatchers, shunting team members, train engineers, and station controllers. It delivers features

such as group calls, voice broadcast, location-based connections, and call pre-emption in case of

an emergency, and supports applications such as cargo tracking, video surveillance in trains and

at stations, and passenger information services.

The RF carrier in GSM can support up to eight Time Division Multiple Access (TDMA)

timeslots. Thereby, in theory, each RF carrier is capable of supporting up to eight simultaneous

telephone calls; but network signaling and messaging may reduce the overall number from eight

timeslots per RF carrier to six or seven timeslots per RF carrier, therefore reducing the number of

mobiles that can be supported.

Unlike a PSTN network, where every telephone is linked to the land network by a pair of fixed

wires, each MS connects to the network over the radio interface only when required. Therefore,

it is possible for a single RF carrier to support many more mobile stations than its eight TDMA

timeslots. It has been found that a typical RF carrier can support up to 15, 20 or even 25 MSs.

Obviously, not all of these MS subscribers could make a call at the same time. Therefore,

without knowing it, MSs share the same physical resources, but at different times.


System Band Uplink Downlink Channels Duplex S pacing

GS M 400

(now obsolete) 450 450.4 - 457.6 460.4 - 467.6 35

GS M 400

(now obsolete) 480 478.8 - 486.0 488.8 - 496.0 35

GS M 850

(Cellular Band) 850 824.0 - 849.0 869.0 - 894.0 124

GS M 900

(P-GS M) 900 890.0 - 915.0 935.0 - 960.0 124 45 MHz

GS M 900

(E-GS M) 900 880.0 - 915.0 925.0 - 960.0 174 45 MHz

GS M-R 900 876.0 - 880.0 921.0 - 925.0 19

GS M 1800

(DCS 1800) 1800 1710.0 - 1785.0 1805.0 - 1880.0 374 95 MHz

GS M 1900

(PCS 1900) 1900 1850.0 - 1910.0 1930.0 - 1990.0 299 80 MHz

Fig. 4: Table showing GSM Frequency Bands

Frequency Re-use

Standard GSM has a total of 124 frequencies available for use. Most network providers are

unlikely to use all of these frequencies due to their small subscriber base and are generally

allocated a small subset of the 124. To overcome this limitation the network provider must re-use

the same frequencies over and over again in different cells over the network, in what is termed a

―frequency re-use pattern‖.


GSM Cell

A cell may be defined as an area of radio coverage from one BTS antenna system. It is the

smallest building block in amobile network and is the reason why mobile networks are often

referred to as cellular networks.

Fig. 5: GSM Cell System

Cell Size and shape

There are two main types of cell:

Omni directional cell: An omni-directional cell (or omnicell) is served by a BTS with an

antenna which transmits equally in all directions (360 degrees).

Sector cell: A sector cell is the area of coverage from an antenna, which transmits, in a

given direction only. As the number of MS increases in the same geographical region, the

number of cells is to be increased to meet the demand. Hence the size of the cell is

decreased to fit more cells into this geographical area. To achieve this, a technique called


―sectorization‖ is employed. Sectorization splits a single site into a number of cells, each

cell having separate transmission and reception antennas and behaving as an independent

cell, using special directional antennas to ensure that the radio propagation from one cell

is concentrated in a particular direction.

The shape of a cell depends upon the number of cells surrounding it. It has been observed that a

minimum number of 6 cells must surround a cell so that the entire area is covered, as well as

adjacent cell interference is minimum. This gives each cell an effective hexagonal shape.

Fig. 6: Basic Cell Shape

Fig. 7: Cell Sectorization


Cell Coverage

The mobile stations connect to the GSM network by searching for cells in the immediate

vicinity. The number of cells in any area is determined by the number of MS subscribers who

will be operating in that area, and the geographic layout of the area. There are five different cell

sizes in a GSM network—macro, micro, pico, femto and umbrella cells.

Macro cells cover large areas. These can cover a maximum radius of 35 km and transmit

power upto 8W. These are generally used in areas having few subscribers or where a

large area is to be covered using minimum number of cells.

Micro cells have antenna height comparable to the average roof top level; typically used

in urban areas. These are used to support a large number of MSs in a small geographic

region, or where a low transmission power is required to reduce the effects of

interference, covering a minimum radius of 200m.

Pico cells are small cells whose coverage diameter is upto 200m; they are mainly used

indoors.

Femto cells are cells designed for use in residential or small business environments and

connect to the service provider‘s network via a broadband internet connection.

Umbrella cells are used to cover shadowed regions of smaller cells and fill in gaps in

coverage between those cells.

The coverage of a MS by a cell depends on the signal strength as well as other factors. E.g. a

MS in a fast moving vehicle will be allocated to a macro cell over a pico cell to minimize

handovers and ensure continuous coverage.

Cell Planning

Cell planning involves the planning as to where the cell sites are to be located and how much

area will they cover. Cell planning begins with traffic and coverage analysis. Data is collected

and analyzed to estimate the geography of the area, as well as the estimated traffic. The data

collected includes:

Coverage area

Estimated traffic, based on demographic and geographic constraints such as:

- Population distribution

- Income level distribution

- Land usage data

Grade Of Service (GOS)

Available frequencies

System growth capability

This data is then analyzed to generate a model of the proposed cell sites and coverage area.


The cell planning has to be done so that interference is at the minimum, while covered area is

maximum possible using minimum number of cells.

Co-channel interference: this occurs when same frequency is used in attached cells,

resulting in interference.

Adjacent channel interference: this is interference caused by extraneous power from

signal in an adjacent channel.

Fig. 8: Map depicting Cell Planning

Cell Clusters

The cells in a GSM network are divided into clusters. Each cluster is a group of cells that uses

different frequencies. Thus, the network is made of repeating clusters of cells. Since clusters are

repeated, and only the cells within a cluster require different frequencies, the number of different


frequencies needed is the number of cells per cluster, i.e., the cluster size. The most common

clusters used are the 4/12 model, and more recently, he 3/9 cluster pattern.

The 4/12 pattern uses 4 BS to cover a total of 12 cells. This cluster of 12 cells is then repeated

over and over in the network. Owing to the large cluster size, adjacent cells have different

frequencies, and there‘s no co-channel interference.

Fig. 9: The 4/12 cell cluster pattern

The 3/9 cluster is a new concept, and uses 3 BS to cover 9 cells. However, here adjacent cells

have comparable frequencies (A1 and C3), hand hence, frequency hopping is required to avoid

co-channel or adjacent channel interference.


Fig. 10: The 3/9 cell cluster pattern

Switching and Control

The MS routinely move from one cell‘s coverage area to another cell‘s coverage area. Handovers

from one cell to another could be for a number of reasons. Regardless of the reason for a

―handover‖ it has to be controlled by the Mobile services Switching Centre (MSC).

To perform a handover, the network must know which neighbour cell to hand the MS over to.

The MS performs measurements of its surrounding neighbour cells and reports its findings to the

network. These are then analyzed together with the measurements that the ne twork performs and

a decision is made on a regular basis as to the need for a handover. If a handover is required then

the relevant signal protocols are established and the handover is controlled by the MSC.


Features of GSM

Compatibility:

The analogue cellular networks during the 1980s had many different cellular systems which were

incompatible with one another. GSM has been specified and developed by many countries

working in co-operation with each other. The result is a cellular system which has been

implemented throughout the world, resulting in compatibility across the globe.

Noise Robust:

Old cellular telephone systems, such as AMPs, used analogue radio signals. Although this

technique provided an excellent audio quality, it was vulnerable to noise due to interference or

lightening etc.

In order to combat the problems caused by noise, GSM uses digital technology instead of

analogue. GSM air interface in RF environments can produce a usable signal, where analogue

systems would be unable to. This leads to better frequency re-use patterns and more capacity.

Flexibility and Increased Capacity

In analogue systems, every connection between an MS and a cell site requires a separate RF

carrier, which in turn requires a separate set of RF hardware. In order to expand the capacity of a

cell site by a given number of channels, an equivalent quantity of hardware must be added. This

makes system expansion time consuming, expensive and labour intensive. Re-configuration

suffers similar problems since much of the equipment requires manual re-tuning and this makes

the system inflexible. Use of digital transmission resolves all these problems.

GSM equipment is fully controlled by its software. Network re-configurations can be made

quickly and easily with a minimum of manual intervention required. Also, since one carrier can

support eight users, expansion can be made with less equipment.

GSM‘s use of a digital air interface makes it more resilient to interference from users on the

same or nearby frequencies and so cells can be packed closer together, which means more

carriers in a given area to give better frequency re-use.

Multi-band networks

Mobiles equipment are available where a user can make use of both the GSM 900 and the

1800/1900 networks. This enables network operators to add in capacity and reduce network

interference by using cells operating in different frequency bands. This means using techniques

like sectorization, micro cells and frequency hopping.

Use of Standardized Open Interfaces


The equipment in each of the analogue cellular networks tends to be produced by one

manufacturer. This is very profitable for the manufacturers as they control the pricing of their

product. Unfortunately for the MS user and the network provider, this means high prices.

The situation is very different with GSM, where standard interfaces are used throughout the

networks such that network planners can select different manufacturers for different components.

Competition between manufacturers is therefore intense in the GSM market.

Improved Security and Confidentiality

Security figures high on the list of problems encountered. Extensive measures have been taken,

when specifying the GSM system, to substantially increase security with regard to both call theft

and equipment theft.

With GSM, both the Mobile Equipment (ME) and Mobile Subscriber are identified. The ME has

a unique number coded into it when it is manufactured. This can be checked against a database

every time the mobile makes a call to validate the actual equipment. The subscriber is

authenticated by use of a smart card known as a Subscriber Identity Module (SIM); again this

allows the network to check an MS subscriber against a database for authentication.

GSM also offers the capability to encrypt all signaling over the air interface. In addition to this,

the GSM air interface supports frequency hopping; this entails each ―burst‖ of information being

transmitted to/from the MS/base site on a different frequency, again making it very difficult for

an observer (hacker) to follow/listen to a specific call.

Enhanced Range of Services

GSM offers a greatly enhanced range of services compared to analogue cellular systems. Apart

from a full range of data transmission options and fax, there are a wide range of supplementary

services. The services available to a subscriber are determined by three factors:

The level of service provided by the network provider.

The level of service purchased by the subscriber.

The capabilities of the subscriber‘s mobile equipment.

Speech Services

The following services listed involve the transmission of speech information and make up the

basic service offered by a network provider:

Telephony

Emergency Calls (with/without SIM Card inserted in MS)

Data Services

Special provision is made in the GSM technical specifications for data transmission. This

includes:


Short Message Service

Facsimile

Advanced Message Handling Service

Dual Personal and Business Numbers

Images, text files

Supplementary Services

A supplementary service is a modification of, or a supplement to, a basic telecommunication

service. The network providers charge extra for these services or use them as an incentive to join

their network. Some of the optional supplementary services offered to GSM subscribers are:

Call Barring

Call Forwarding

Call Completion

Multi-party calls

ISDN Compatibility

Integrated Services Digital Network (ISDN) is a standard and advanced telecom network

designed to carry voice and user data over standard telephone lines.

The GSM network has been designed to operate with the ISDN system and provides features

compatible with it. GSM can provide a maximum data rates 9.6 kbit/s while ISDN provides

much higher data rates than this (standard rate 64 kbit/s).This is due to the fact that the ISDN

provides three effective connections, one for signaling, and two others for data and speech

transmission, while GSM offers just a single one. Thus ISDN is much faster than GSM


GSM Generations

The GSM system has come a long way since its inception in the late 1980‘s. There have been

three main generations of GSM as:

1G: This was the basic system that was developed in the latter half of the 1980s, and was

launched as a Europe-wide system in 1991, and adopted throughout the world. It

introduces basic services such as:

Voice telephony

International roaming

Basic fax/data services (up to 9.6 kbits/s)

Call forwarding

Call barring

Short Message Service (SMS)

2G: This phase of development involved additional features as:

Advice of charge

Calling line identification

Call waiting

Call hold

Conference calling

Closed user groups

Additional data communications capabilities

2.5G: This phase involves introduction of many new services over 2G, the most

important being General Packet Radio Service (GPRS). Some of the enhancements

offered by Phase 2+ include:

Multiple service profiles

Private numbering plans

Interworking with GSM 1800, GSM 1900 standards.

Data rates up to 114kbit/s


2.75G: This generation involves introduction of 8 PSK, encoding – Enhanced Data rates

for GSM Evolution (EDGE), which though included in the definition of 3G, is considered

to be separate. This technology allows faster data transfer rates and up to three-fold

increase in capacity.

3G: The International Telecommunication Union (ITU) defined the third generation (3G)

of mobile telephony standards – IMT-2000 – to facilitate growth, increase bandwidth,

and support more diverse applications such as:

Video call

Enhanced security

Faster data rates (up to 14.4 Mbit/s)

4G: This technology is still under development and testing phase, although there have

been trial runs in Japan and the US. It is expected to be ready for public usage by 2010,

and offers:

Higher network capacity

Faster data rates up to 10Gbit/s, with minimum being 100Mbit/s

IPv6 compatibility

Global roaming across multiple networks and different technologies.

Fig. 11: GSM Evolution


GSM Network Components

The diagram shows a simplified GSM network. Each network component is designed to

communicate over an interface specified by the GSM standards. This provides flexibility and

enables a network provider to utilize system components from different manufacturers.

Fig. 12: GSM Network Components

The components are discussed in detail in the forthcoming sections.


Mobile Station (MS)

The MS consists of two parts, the Mobile Equipment (ME) and an electronic ‗smart card’ called

a Subscriber Identity module (SIM).

The ME is the hardware used by the subscriber to access the network. It has an identity number

associated with it, which is unique for that particular device and permanently stored in it. This

identity number is called the International Mobile Equipment Identity (IMEI) and enables the

network operator to identify mobile equipment which may be causing problems on the system.

The SIM is a card which plugs into the ME. This card identifies the MS subscriber and also

provides other information regarding the service that subscriber should receive. The subscriber is

identified by an identity number called the International Mobile Subscriber Identity (IMSI).

The SIM is specific to a particular network and must be obtained from the network provider.

Without the SIM inserted, the ME will only be able to make emergency calls. By making a

distinction between the subscriber identity and the ME identity, GSM can route calls and

perform billing based on the identity of the ‗subscriber‘ rather than the equipment or its location.

Mobile Equipment (ME)

The ME is the only part of the GSM network which the subscriber will

really see. The ME is identified by means of a classmark. The

classmark is sent by the ME in its initial message to the MSC. The

following pieces of information are held in the classmark:

Revision Level

RF Power Capability

Ciphering Algorithm

Frequency Capability

Short Message Capability

Fig. 13: Mobile Equipment

Subscriber Identity Module (SIM)

The SIM is a ―smart card‖ which plugs into the ME and contains information about the MS

subscriber hence the name Subscriber Identity Module.


The SIM contains several pieces of information:

International Mobile Subscriber Identity (IMSI)

Temporary Mobile Subscriber Identity (TMSI)

Location Area Identity (LAI)

Subscriber Authentication Key (Ki)

Mobile Station International Services Digital Network (MSISDN)

Most of the data contained within the SIM is protected against reading or alterations. Some of

the parameters are continuously updated to reflect the current location of the subscriber.

Fig. 14: SIM Card

The SIM card, and the high degree of inbuilt system security, provides protection of the

subscriber‘s information against fraudulent access. SIM cards are designed to be difficult to

duplicate. The SIM can be protected by use of Personal Identity Number (PIN) password.

The SIM is capable of storing additional information such as accumulated call charges. This

information is accessible to the customer via handset/keyboard key entry. The SIM also executes

the Authentication Algorithm whenever the MS is switched on.


Base Station System (BSS)

The GSM Base Station System is the place where all the functions related to radio transmission

and reception are carried out. This includes the equipment located at a cell site. It comprises a

combination of digital and RF equipment. The BSS provides the link between the MS and the

MSC. The BSS communicates with the MS over the digital air interface and with the MSC via 2

Mbit/s links.

The BSS consists of three major hardware components:

Fig. 15: BSS Components

Base Station Controller (BSC)

The BSC provides the control for the BSS. The BSC is basically a high capacity switch that

communicates directly with the MSC. Any operational information required by the BTS will be

received via the BSC. Likewise any information required about the BTS (by the OMC etc.) will

be obtained by the BSC. The BSC incorporates a digital switching matrix, which it uses to

connect the radio channels on the air interface with the terrestrial circuits from the MSC. The


switching matrix also allows the BSC to perform ―handovers‖ between radio channels on BTSs,

under its control, without involving the MSC.

Base Transceiver Station (BTS)

The BTS contains the RF components that provide the air interface for a particular cell. This is

the part of the GSM network which communicates with the MS. The antenna is included as part

of the BTS. Each BTS can support 1 or more cells.

The BTS also has a limited amount of control functionality, reducing the amount of traffic

passing between the BTS and BSC. Where the BSC and BTS both control a function, the control

is divided between the two, or may be located wholly at one. A BSC may control several BTSs.

The BTSs and BSC may either be located at the same cell site (co- located), or located at different

sites (remote). In reality most BTSs will be remote, as there are many more BTSs than BSCs in a

network. Another BSS configuration is the daisy chain. A BTS need not communicate directly

with the BSC which controls it, it can be connected to the BSC via a chain of BTSs. Daisy

chaining reduces the amount of cabling required to set up a network as a BTS can be connected

to its nearest BTS rather than all the way to the BSC.

Transcoder (XCDR)

The transcoder is used to compact the signals from the MS so that they are more efficiently sent

over the terrestrial interfaces. Although the transcoder is considered to be a part of the BSS, it is

very often located closer to the NSS (MSC) to allow more efficient use of the terrestrial links.

The transcoder is used to reduce the rate at which the traffic (voice/data) is transmitted over the

air interface. The XCDR is required to convert the speech or data output from the MSC (64

kbit/s PCM), into the form specified by GSM specifications for transmission over the air

interface, i.e. between the BSS and MS (64 kbit/s to 16 kbit/s and vice versa). The 64 kbit/s Pulse

Code Modulation (PCM) circuits from the MSC, if transmitted on the air interface without

modification, would occupy an excessive amount of bandwidth. The required bandwidth is hence

reduced by processing the 64 kbit/s circuits so that the transfer rate falls to 16 kbit/s.

The transcoding function may be located at the MSC, BSC, or BTS. The content of the 16 kbit/s

data depends on the coding algorithm used. For data transmissions the data is not transcoded but

data rate adapted from 9.6 kbit/s (4.8 kbit/s or 2.4 kbit/s may also be used) up to a gross rate of

16 kbit/s for transmission over the terrestrial interfaces.


Network Switching Subsystem (NSS)

The Network Switching Subsystem includes the main switching functions of the GSM network.

It also contains the databases required for subscriber data and mobility management. Its main

function is to manage communications between the GSM network and other telecommunications

networks. The components of the Network Switching Subsystem are listed below:

Mobile Services Switching Centre (MSC)

Home Location Register (HLR)

Visitor Location Register (VLR)

Equipment Identity Register (EIR)

Authentication Centre (AUC)

Interworking Location Register (ILR)

Fig. 16: NSS Architecture

Apart from the above, the NSS also consists of:

Echo Canceller (EC)

Message Center (MC)

Data Transmission Interface (DTI)


Mobile Services Switching Centre (MSC)

The MSC is included in the GSM system for call-switching, as well as helps in handling SMS

and other services. Its purpose is the same as that of any telephone exchange.

The MSC carries out several different functions, such as:

Call Processing

Operations and Maintenance Support

Internetwork Interworking

Billing

Home Location Register (HLR)

The HLR is the centralized network reference database for subscriber parameters. Various

identification numbers and addresses, as well as authentication parameters are stored here

permanently. This information is entered into the database by the network provider when a new

subscriber is added to the system.

The data it contains is remotely accessed by all the MSCs and the VLRs in the network and there

is only one database record per subscriber. The subscriber data may be accessed using either the

IMSI or the MSISDN number. Often, the HLR is mirrored for better reliability and speed.The

information stored includes:

Subscriber identity (i.e. IMSI, MSISDN)

Subscriber supplementary services

Subscriber location information (i.e. MSC service area)

Subscriber authentication information

The primary functions of the HLR include:

Subscription database management: as a database, the HLR must be able to process data

quickly in response to data retrieval and update requests from other network nodes. For

this reason it acts as a database management system. Each subscriber record contains a

substantial amount of parameters.

Communication with MSC: when setting up calls to an MS, it is necessary for the HLR to

contact the MSC serving the MS for routing information. By analyzing the MSISDN,

MSC knows which HLR to contact worldwide for that MS‘s subscription.

Communication with GMSC: during call set-up to an MS, the GMSC requests MS

location information from the HLR, which then provides this in the form of routing


information. Also, if the subscriber is detached, the HLR will inform the GMSC that

there is no need to perform further routing of the call.

Communication with AUC: before any activity involving subscription information takes

place, the HLR must retrieve new authentication parameters from an AUC.

Communication with VLR/ILR: when an MS moves into a new MSC service area the

VLR for that area requests information about the MS from the HLR of the subscriber.

The HLR provides a copy of the subscription details, updates its MS location information

and instructs the old VLR to delete the information it has about that MS. As the ILR acts

as a VLR for AMPS subscribers, the HLR communicates with it in a similar way.

Visitor Location Register (VLR)

The VLR contains a copy of the data stored at the HLR. It is temporary data which exists as long

as the subscriber is ―active‖ in the area covered by the VLR. The VLR provides a local database

for the subscribers wherever they are physically located within a PLMN. This eliminates the

need for excessive references to the HLR database. The data stored in the VLR is:

Mobile Station Roaming Number (MSRN)

Mobile status (busy/free/no answer etc.)



Mobile Subscriber Roaming Number (MSRN)

The VLR can allocate a Mobile Station Roaming Number (MSRN) for subscribers outside

their home network. This number is assigned from a list held at the VLR. The MSRN is then

used to route the call to the MSC which controls the base station in the MS‘s current location.


Cells within the Public Land Mobile Network (PLMN) are grouped together into

geographical areas. Each area is assigned a Location Area Identity. Each VLR controls

several LAIs and as a subscriber moves from one LAI to another, it is updated in the VLR.


The VLR controls the allocation of new Temporary Mobile Subscriber Identity (TMSI)

numbers and notifies them to the HLR. The TMSI is updated frequently, making it difficult

for the call to be traced and therefore providing a high degree of security for the subscriber.

The TMSI may be updated in any of the following situations:

Call setup. On entry to a new LAI.

On entry to a new VLR.

Following occurs when MS‘s move into a new service area:


The VLR checks its database to determine whether or not it has a record for the MS

(based on the subscriber‘s IMSI).

When the VLR finds no record for the MS, it sends a request to the subscriber‘s HLR for

a copy of the MS‘s subscription.

The HLR passes the information to the VLR and updates its location information for the

subscriber. The HLR instructs the old VLR to delete the information it has on the MS.

The VLR stores its subscription information, including the latest location and status.

Equipment Identity Register (EIR)

The EIR contains a centralized database for validating the International Mobile Equipment

Identity (IMEI) number of the Mobile Equipment being used. This database is concerned solely

with MS equipment. The EIR database consists of lists of IMEIs (or ranges of IMEIs) organized

as follows:

White List Contains the IMEIs of MSs known to have been assigned to valid MS equipment.

Black List

Contains IMEIs of MS which have been reported stolen or which are to be denied service

for some other reason.

Grey List

Contains IMEIs of MS which have problems (for example, faulty software). These are

not, however, sufficiently significant to warrant a ‗‗black listing‖.

The EIR database is remotely accessed by the MSCs. A network may contain more than one EIR

with each EIR controlling certain blocks of IMEI numbers. The MSC contains a translation

facility, which when given an IMEI, returns the address of the EIR controlling the appropriate

section of the equipment database, which is then contacted to determine the authenticity o f the

equipment.

Authentication Centre (AUC)

The AUC is a processor system; it performs the ―authentication‖ function. It is normally co-

located with the Home Location Register (HLR) as it has to continuously access and update the

subscriber records. The AUC/HLR centre can be co- located with the MSC or remotely. The

authentication process usually takes place each time the subscriber ―initializes‖ on the system,

and involves validation of the MS so that it may make/receive calls. Also, ciphering of the

signals to the particular MS is performed by this unit for security purposes.


Interworking Location Register (ILR)

The ILR enables the GSM system to interface with the various forms of public and private data

networks currently available. Its major function today is to provide interconnectivity between the

various consumers operating on different bands, such as the GSM900 and DCS1800 band. This

unit also facilitates interconnectivity between the GSM and CDMA systems.

The ILR generate the Inter- Working Functions (IWF), that carry out the actions. The basic

features of the IWF are listed below.

Protocol conversion Data rate adaption

Some systems require more ILR capability than others; this depends upon the network to which

it is being connected.

Echo Canceller (EC)

An EC is used on the PSTN side of the MSC for all voice circuits. Echo control is required at the

switch because the inherent GSM system delay can cause an unacceptable echo condition, even

on short distance PSTN circuit connections.

During a normal PSTN land to land call, no echo is apparent because the delay is too short and

the user is unable to distinguish between the echo and the normal telephone ―side tone‖.

However, without the EC and with the GSM round trip delay added, the effect would be very

irritating to the MS subscriber, disrupting speech and concentration.

Data Transfer Interface (DTI)

The DTI implements the GSM Inter-Working Function (IWF). It performs data handling

functions such as data rate conversion and provides the functions necessary for data interworking

between GSM networks and other networks, including:

Data Traffic to/from PSTN: this involves modem and fax calls. For connections to the

PSTN a modem is selected by the DTI to perform the necessary rate and format

conversions.

Data Traffic to/from ISDN: the whole set of data communications towards ISDN is

available, since the MSC/DTI is capable of signaling and mapping basic service

information between the ISDN and the GSM network.

Data Traffic to/from PDNs: the DTI handles data traffic to and from Public Data

Networks (PDNs) such as the Packet Switched PDN (PSPDN) and Circuit Switched PDN

(CSPDN).


Data Traffic between mobiles: the data traffic inside the PLMN must pass through the

DTI to handle the protocol used for rate adaptation in the radio path.

HSCSD: this version of High Speed Circuit Switched Data (HSCSD) allows the

connection of 2, 3, or 4 time slots on one radio channel each carrying 9.6 kbits/s. The

DTI handles rate conversion to PSTN or ISDN as appropriate.

Message Center (MC)

An MC may be added to a GSM network to provide one or more of the following messaging

services:

Voice mail

Fax mail

Short Message Service (SMS) text messages

SMS Cell Broadcast (SMSCB) text messages

These services generate considerable revenue for the network operator, as they are becoming

increasingly popular.

Voice Mail

Voice mail ensures that all calls to a person can be completed, even when a person does not

answer calls. The calling party can record a voice message for the subscriber they are calling.

A subscriber can use their MS to select diversion to voice mail based on a particular event or

status (e.g. busy, unreachable). The subscriber is informed that they have voice messages in

their mailbox by means of either a short text message or phone call from the network at

regular intervals. If their MS is detached, this indication is sent when the subscriber next

attaches to the network. The subscriber can then retrieve their voice mail messages at a later

stage. Functions for storing voice messages over a long period also exist.

Fax Mail

Fax mail operates similarly to voice mail. For MS that support fax, a subscriber can set

diversion for all or some fax calls to a fax mailbox. When the MS is next attached to the

network, the network will deliver the fax message to a fax machine identified by the MS.

SMS

A short text message consists of up 160 alphanumeric characters, entered at a Short Message

Entity (SME) such as an MS (using the keypad) or computer terminal. A short message

always originates or terminates in a GSM network, meaning that a short message cannot be


sent between two SMEs residing outside a GSM network. The short message originator

knows if the message delivery is successful or unsuccessful via notification. When a message

is submitted, the deferred delivery option can be requested. This option makes it possible to

specify the time the message is to be delivered.

An MC, which handles SMS messages, is often referred to as an SMS Center (SMS-C).

When a message is to be forwarded to an MS, the system must first determine where the MS

is situated. As in ordinary voice traffic, a gateway requests the routing information. The

gateway is called the SMS GMSC.

Each short message is time stamped by the SMS-Center when it is submitted. A message is

deleted once the delivery is successful or once the time specified in deferred delivery expires.

SMSCB

The SMSCB service enables a message of up to 93 alphanumeric characters to be delivered

to all attached MS‘s in one cell. This may be useful for identifying key phone numbers in the

cell‘s area such as that of a hospital or police station. Alternatively, it may be used for

advertising services within the cell.

These are generally utilized for advertising, or for relaying important information to many

people in one go.


Mobile Station Switching Centre (MSC)

MSC stands for ―Mobile station Switching Center‖, and is the heart of the GSM network. This

element is similar to any PSTN exchange. It performs call switching, which means connecting,

maintaining and release of mobile to mobile, mobile to PSTN and PSTN to mobile calls.

1. Services:-

MSC has the database of all the subscribers in the network. The subscriber database includes the

subscriber numbers and identities like IMSI, MSISDN etc. All the BSC's in the network are

connected to the MSC .MSC connects all the calls coming and going out to the PSTN through

the BSC's. Apart from these facilities, it provides the following three types of services:-

1.1 Teleservices

A teleservice provides the transmission of information between two terminals including the application protocols of the latter. Currently, the following teleservices are supported :

Telephony. This service allows a mobile subscriber to receive and to set up a normal speech call.

Emergency Call. This service is used to set up an emergency call to an emergency call center. An emergency call can be originated by pushing the emergency button at the mobile station, dialing the

international emergency number 112, or dialing a PLMN-dependent emergency number, even if the subscriber identity module (SIM) is not inserted.

Short Message Service. This service enables a mobile subscriber to receive or send a short alpha-numeric message.

Alternate Speech/Facsimile Group 3 Transparent.

This service allows a mobile subscriber to switch between telephony and the facsimile group 3 transparent service.

1.2 Bearer Services A bearer service is used for transporting user data in form of synchronous or asynchronous

data or speech at speeds of 300-9600 bps.

1.3 Supplementary services

These services are optional and the may be provided for free, for a charge, or not at all:

CLIP (Calling Line Identification Presentation)

The CLIP service gives a mobile subscriber the directory number of the calling subscriber.

Upon activation, the number of the caller is sent to the MS if the caller does not have a CLIR

service activated for the dialed number to prevent display of number. However, a CLIR

override may be used in this case to display the number.


CLIR (Calling Line Identification Restriction)

CLIR gives the mobile subscriber authorization to suppress the display of his directory

number for the called subscriber. The information on this restriction is sent to the called

subscriber MSC or PSTN exchange. The permanent CLIR service suppresses presentation of

the number for every call except if the called subscriber has an authorization like CLIP

override. With the temporary service it is possible to decide for each individual call whether

or not the presentation is to be suppressed.

Call Waiting (CW)

Call Waiting enables a mobile subscriber to be informed of another incoming call request

during an existing connection. The existing connection must be for the service ―Telephone‖,

while the call request may be for any service.

Call Hold (CH)

The supplementary service ―call hold‖ enables a mobile subscriber to interrupt and later

continue an existing speech connection. During this interruption, the radio channel is still

assigned to the mobile subscriber and can be used for another connection.

Multiparty service (MPTY)

This service enables a mobile subscriber to set up and control voice connection to multiple

subscribers simultaneously. In such a call, all the parties can interact simultaneously, the

prerequisite being that the MS initiating the call also has the facility ―call hold‖

Advice of Charge : In this service, with each call data are sent from the MSC to the MS which enables it to

calculate independently the charge units accruing in the course of the call.

Call Barring

This enables a subscriber to block a certain category of outgoing or incoming calls or to have

them barred by the operator in the HLR. The following type of calls can be barred:-

- all outgoing calls

- all outgoing international calls - all outgoing international calls, except those to the mobile subscriber‘s home PLMN

country. - all incoming calls - all incoming calls while the subscriber is roaming outside the home PLMN

Call Forwarding

This service enables a called mobile subscriber to forward incoming calls to another number.

The call is of following types:

- call forwarding unconditional - call forwarding on busy - call forwarding on no reply

- call forwarding on MS not reachable

Apart from these services, the MSC also performs the following functions: - Control of base station sites and providing radio subsystem access to the subscriber and

equipment databases.


- Providing the interface between the fixed and mobile network. In this role it is known as the Gateway MSC (GMSC), since it gives mobile network access to the land lines of the

PSTN, ISDN and data network. Thus acting as an exchange for all type of mobile calls.

2. Functions of MSC:-

The function of the MSC is to act as the “brain” of the NSS, and maintain coordination among

the various components, and to route the requests to the relevant section for processing. It helps

in:

IMEI check

Location Updates

Authentication

Interrogation

Handover

Call Routing

3. Switching Network (SN)

The Digital Electronic Switching System (EWSD) is equipped with a very powerful switching

network (SN). By virtue of its high data transmission quality, the switching network can switch

connections for various types of service like telephony, facsimile, teletext, data transmission. In a

network node, switching network is the link between the following:

- Line trunk groups (LTGs) for speech and data connections.

- LTGs and coordination processor (CP) for message exchange.

- LTGs and common channel signaling network control (CCNC) for common channel

signaling system no. 7 (CCS7) message exchange.

Besides switching calls, the SN also switches the connections between the line/trunk groups and

the coordination processor (CP). These connections, for the exchange of control data, once set up

remain constantly available. They are therefore referred to as ―Semipermanent Connections

(SC)”. The line/trunk groups use these connection paths for exchange of messages without

imposing any load on the coordination processor. The SC can also be set up for private lines or

for common channel signaling.

3.1 Features

The most significant features of the SN are:

- low space requirement

- negligible internal blocking

- high degree of functional integrity provided by duplication

- ease of expansion

- use of the latest technology (NMOS and TTLLS)

- easy expansion


- one switching format for both speech and data signals (octets)

- single-channel connections

- broadcast connections available for application of signal sources

- self-supervision

3.2 External Interfaces

to the line/trunk groups (LTG)

Here, one time slot (slot 0) is for communication between the LTG and the CP, while 127

(slots 1-127) are for the subscriber connections.

to the common channel signaling network control (CCNC)

Common channel signaling (CCS) information is exchanged.

to the coordination processor (CP)

The commands and messages are sent to the CP.

to the switch group control (SGC)

Commands from the CP to an SGC and messages from an SGC to the CP are transferred.

Just as the switching network is duplicated, so is each of the four above-mentioned inter-

faces. At these interfaces, the two directions of transmission are screened from one

another but carried in the same cable. There is one secondary digital carrier for

information, one exchange clock line and one frame mark bit line for each direction of

transmission.

4. Line trunk groups (LTGs)

LTG stands for Line Trunk Group. It is a hardware part of MSC. One LTG contains four DLUs

(Digital Line Unit). Each DLU is used to terminate one PCM either from BSC or from PSTN

side.

4.1 Types of LTGs

According to hardware point of view the LTGs can be divided into three types:

1) LTG of type B (LTG B) 2) LTG of type M (LTG M)

3) LTG of type N (LTG N)

The three types of LTG differ from each other only in the number of hardware components

used for one LTG.


LTG B :-

In this LTG there is:-

One card for each DIU i.e. 4 DIU cards.

One card for Processor.

One DEC card (for PSTN DIU).

LTG M:-

In this LTG there is:-

One card for 4 DIUs, i.e. one DIU card.

One card for Processor.

One DEC card (for PSTN DIU). LTG N:-

It is the latest type of LTG. It contains only

One card for four DIU and Processor

One DEC card.

According to software configuration point of view, LTGs can be of two types. These are:-

1) LTG of type 41 (LTG 41)

2) LTG of type 46 (LTG 46)

If during software configuration of LTGs we define a LTG for PSTN side then it called LTG

46 but if it is defined for BSC side then it is called as LTG 41. Any of the LTGs can defined

as of type 46 or 41.


Registration of the MS on the Network

The network conducts certain checks whenever a new MS enters it, or when a MS request to

make/receive a call, and the MS failing any of these tests is not allowed to make/receive calls.

These involve basic housekeeping operations like certifying the MS, pupation its location, etc.

The processes involved are:

IMEI Check

Authentication

Location Update

Interrogation

Handover (If required)

Fig. 17: MS registration process

1. IMEI Check

Here the IMEI number of the ME is checked with the EIR to decide whether it is white- listed,

black-listed, or grey- listed. This is usually done before Location Updates, or call completion.

The sequence of IMEI check is as below:-

1. The MSC/VLR requests the IMEI from the MS.

2. MS sends IMEI to MSC.


3. MSC/VLR sends IMEI to EIR.

4. On reception of IMEI, the EIR examines three lists:

– A white list containing all equipment identities that have been to valid equipment.

– A black list containing all equipment identities that have been barred.

– A gray list (on operator level) containing faulty or non-approved mobile equipment.

5. The result is sent to MSC/VLR, which then decides whether or not to allow network

access for the terminal equipment.

- If the IMEI is white- listed, access is granted.

- If the IMEI is grey-listed, access is granted, but the operator is informed about the

detection of grey listed mobile equipment. An observed ME record is generated.

- If the IMEI is black-listed or unknown, access is denied. The operator is informed about

the detection of black listed mobile equipment. A ME record is generated. Emergency

calls made on black listed or unknown mobile equipment are not terminated unless

project specific settings indicate that they should be terminated.

Fig. 18: IMEI check procedure

2. GSM Authentication Process

Authorization of a MS takes place whenever the MS is switched on, or when it enters a

network area other than its home network, and it is only after authentication that the MS can

avail of the services provided by the network, i.e. when the SIM has been certified as

authentic.

The following data is used in this process:

RAND = randomly generated number.


A3, A5, A8 = Manipulation algorithms defined on allocation of IMSI.

Ki = Authentication key stored in the SIM.

SRES = Signed Response. Derived from A3 (using RAND, Ki).

Kc = Cipher Key. Derived from A8 (using RAND, Ki). (Also called encryption key, Ke)

The process of the authentication involves:

Kc, SRES and RAND are stored at the VLR.

The AUC sends RAND to the MS.

The MS, using the A3 and A8 algorithms and the parameter Ki stored on the SIM card,

with the received RAND from the AUC, calculates the values of SRES and Kc.

The MS sends SRES to the AUC.

Within the AUC the value of SRES is compared with the SRES received from the

mobile. If the two values match, then the authentication is successful.

Using Kc, encryption between the MS and the BSS can now occur over the air interface.

The first time a subscriber attempts to make a call, the full authentication process takes place.

However, for subsequent calls attempted within a given system control time period, or within

a single network, authentication may not be necessary.

3. Location Update

The process of informing the MSC about new Location Area is called Location Update (LU).

The MSC/VLR performs a location update to load the mobile subscriber data in the VLR

database and to adapt the location information. The procedure is initiated after the mobile

subscriber has moved in the area controlled by the MSC/VLR.

The procedure of Location Update (LU) is:-

A LU is initiated by the mobile when it enters a new Location Area. The LA is

transmitted on the BCCH (Broadcast Control Channel) as the LAI (Location Area

Identity). The mobile is assigned an SDCCH (Stand-alone Dedicated Control Channel)

by the BSS; the Location Updating will be carried out using this channel.

Once the SDCCH has been assigned the mobile transmits a LU request. The MSC then

sends the new LAI and current TMSI number to the VLR. The information is also sent to

the HLR if the mobile has not previously updated on the network.

Authentication and ciphering may now take place if required.

The VLR now assigns a new TMSI for the mobile; this number is sent to the MSC; and

initiates the LU accept, which transmits the new TMSI and LAI to the mobile.

Once the mobile has stored both the TMSI and the LAI on the SIM card it sends the


acknowledgement to the MSC, which informs the VLR.

The SDCCH will then be released by the mobile.

3.1 Types of Location Update:-

- Normal Location Update (Explained above)

- IMSI Attach: In this case, the ME, when turned off, sends an IMSI detach message to the

MSC. When it is turned on, it compares its LAI to that provided by the network, and

sends an IMSI attach to the MSC, else requests a location update.

- Periodic Location Update: This is done periodically by the MSC to update the VLR and

HLR. However, if the MS enters a non-coverage zone, the MSC would be reduced to

sending page after page without a response from the MS, wasting power and jamming

channels. Here, if the MS does not reply to the MSC within a set period, the MSC

detaches the IMSI of the MS, and rejects all incoming calls to it. This is called Periodic

Location Update.

4. Interrogation

Since the MS location is contained in only in the HLR database, this information has to be

interrogated for each MTC (Mobile Terminated Call) in order for the connection to be set up

to the mobile subscriber. The whole procedure for ascertaining the routing data for an MTC

is referred to as Interrogation, always initiated by an MSC.

Interrogation comprises the following steps:-

- A connection request for a MTC arrives in an MSC; the MSISDN is therefore available

as the called mobile subscriber's address and identification.

- The MSC carries out global title translation to obtain the signaling point code of the

called subscriber's HLR. The MSC sends the routing data request to the HLR.

- The signaling and address of the VLR is ascertained by the HLR through a global title

translation and an MSRN request is sent to the VLR. The VLR replies to this request by

sending the MSRN.

- The HLR forwards the MSRN to the MSC.

- The MSC performs a digit translation with the MSRN to conclude routing to the location

MSC.

5. Handover

Handover is the process by which the control/communication of a MS is transferred from one

cell to another. In other words, HO is the procedure which preserves the call connection as the

MS moves from one radio coverage area to another. Handover sequence is explained below:-


The MS continuously compiles measurements both of the current transmission and the

broadcast control channels of upto 32 surrounding cells The measurements from the six

best cells are reported back to the BSS every 480 ms.

When HO is required, due to low Receive Signal Strength Indication (RSSI) or poor

signal quality the existing "originating BSS (oBSS)" notifies the MSC "HO required".

The target or new BSS (nBSS) is sent the message "HO Request" tagged with TMSI.

The nBSS allocates a HO Reference number which it uses to determine whether the

correct mobile gains access to the air interface channel which it allocates, and

acknowledges the MSC's request with "HO Request Ack". This is tagged with the "HO

Reference No.". The new BSS assigns a channel to the MS.

The MSC via BSS orders the MS to change to the new channel with the message "HO

Command" on FACCH.

There is an information interchange between new BSS and old BSS .This uses the

FACCH channel. The information carried depends upon the type of HO being performed.

On completion of information transfer, the message "HO Complete" is sent to the MSC.

The MSC sends a "Clear Command" to the BSS; freeing radio resources for another MS.

The MS now sends the periodic measurement reports to the new BSS.

4.1 Types of Handover:-

Intra-BSC Handover:-It takes place if the cell to which handover is to be done belongs to

the same BSC, which handles everything without involving MSC. The MSC is informed by

the BSC after the handover. It is of 2 types:

- Intra-Cell Handover

It is a handover between channels or timeslots of same cell.

- Inter-Cell Handover

It is a handover between different cells of the same BTS

Inter-BSC Handover:-In this type of handover, the mobile is handed over to the cell which

belongs to another BSC. The MSC is completely involved in this handover.

Inter-MSC Handover:-In this case, the cell belongs to another MSC. Here, the handover

takes place through the interconnecting element (PSTN) between the MSC's.


GSM Call Procedure

The figure below shows the basic components and processes involved in setting up a call

between a GSM MS and an ordinary ―land‖ telephone. However, the processes inherent in the

initiation and completion of the call are considerably more complex.

Fig. 19: GSM call procedure

From the subscriber‘s point of view, there are two different call setup procedures requiring a

different approach.

Mobile originated call (MOC)

Mobile terminated call (MTC)

The mobile to mobile call (MMC) may be considered to be a combination of both the MOC and

the MTC.


Mobile Originated Call (MOC)

If the mobile subscriber initiates the call, we talk about a mobile originated call (MOC). Then,

the mobile subscriber is the calling party. Such calls include speech/data calls, emergency calls,

subscriber controlled inputs and short messages. For subscriber controlled inputs and short

messages, no traffic channel is required.

Fig. 20: MOC initiation and completion

1) The MS requests a signaling channel.

2) The BSC grants a channel.

3) The MSC/VLR is informed of the incoming request. If required, it initiates

authentication, ciphering, TMSI reallocation and subscription check to verify the

subscriber‘s access to the service.

4) The MSC/VLR allocates a traffic channel to the MS via the BSC. The dialed digits are

analyzed to determine the destination of the call. The MSC/VLR forwards the initial

address message to the next exchange if required.

5) The MSC informs the PSTN about the call request, which then establishes a connection

to the subscriber.

6) The called party answers and the MSC/VLR is informed by means of the answer

message. The call is completed towards the MS which acknowledges the through-

connection. From that moment on, the MSC/VLR starts charging.

Mobile Terminated Call (MTC)

If the subscriber receives the call, he is involved in a mobile terminated call (MTC). Then, the

mobile subscriber is the called party. Here the processes are handled by the servicing MSC and

VLR, but the Gateway MSC might also be involved. This includes speech/data calls and short

messages to the mobile subscriber.

1. A PSTN/ISDN subscriber dials an MSISDN number. Using digit translation, the call is

routed to the GMSC which interrogates the subscriber‘s HLR.


Fig. 21: MTC initiation and completion

2. The GMSC analyses the MSISDN and finds the HLR identity (by means of the global

title translation function) and sets up a signaling connection to the HLR and requests

routing information (message: send routing information).

3. Upon receiving the request, the HLR searches the MS record and figures out the area the

subscriber is currently located in. Also, it performs subscription and compatibility checks.

4. By means of the VLR-ISDN number, the HLR sets up a signaling connection to the VLR

and requests a roaming number (message: provide roaming number).

5. The VLR performs subscription and compatibility checks and allocates an MSRN. This

MSRN is returned to the HLR and forwarded to the GMSC. At the same time, the VLR

starts a timer for supervision purposes.

6. The GMSC replaces the MSISDN with the MSRN and restarts the digit analysis to route

the call further to the visited MSC/VLR.

7. The visited MSC recognizes the MSRN and informs the VLR of an incoming call

attempt. The VLR releases the MSRN for future use and resets the supervision timer, and

then consults its database to determine the location area where the mobile subscriber is

located in.

8. It sends a paging request to all base stations covering this location area. The base stations

then broadcast the paging request over the complete area.


9. The MS answers with a paging response and radio resource and mobility management

connections are set up.

10. The VLR is notified about successful paging and, if required, initiates authentication,

ciphering and TMSI reallocation.

11. The MSC/VLR starts the call control establishment by sending the setup message to the

mobile station. Later, the MSC/VLR allocates a traffic channel and seizes a terrestrial

circuit. Equipment control is performed in parallel.

12. If the mobile subscriber answers the call, the MSC/VLR informs the originating network

by means of the answer message and completes the call towards the mobile station. From

that moment, the MSC/VLR starts charging the calling party.

Mobile-to-Mobile Call (MMC)

A mobile-to-mobile call (MMC) is a call between two mobile subscribers. The MMC may be

considered an MOC followed by an MTC. The MOC initiation and completion procedures are

then handled by the originating-MSC/VLR and the MTC initiation and completion procedures

are handled by the destination-MSC/VLR. The originating-MSC/VLR may also be responsible

for the interrogation procedure.

When the two subscribers are in the same MSC/VLR area and the traffic channels are switched

entirely within this one MSC/VLR, we speak about a ―mobile internal call (MIC)”. Here, the

servicing MSC/VLR performs both the MOC and MTC initiation/completion procedures. HLR

interrogation is still necessary and, for an MIC, it is carried out by the servicing MSC/VLR itself.

Call Charge Registration

Charging records can be generated for all mobile originated and mobile terminated calls.

These MOC/MTC charging records are generated at the visited MSC/VLR for the calling/called

mobile subscriber. The charging records include date and time, chargeable time interval, type of

telecommunication service (teleservices or bearer services), call or igination or destination and

the invocation of supplementary services. After generation the records are stored within a file,

waiting for a transfer to a billing center (i.e. post processing center) where the subscriber bills are

generated.


Conclusion

The internship at Idea Cellular Ltd. facilitated me in understanding the intricacies of the

GSM System, as well as the working of the Network Switching Subsystem, which is the

backbone of the GSM network for any operator. The internship was a rewarding experience

in which I got to learn a lot about the working of the cellular network.

The network set up by Idea Cellular Limited is among the best in the world with one of the

best customer satisfaction rates being reported by Idea Consumers in opinion polls. The

incidences of network congestion and call dropping rates are one of the lowest in the

contemporary Indian telecom industry that features global giants like Vodafone and Bharti

Airtel.

At Idea Cellular Limited, I was provided practical training about the structure and working

of the GSM network and the associated systems and processes. However, owing to time

constraints, I was unable to study the working of the Object Modeling Technique (OMT)

divis ion, which is a computerized tool for dealing with customers and also controls the

working of the NSS, recording the call durations and calculating the call charges among

others.


References

1. www.ideacellular.com

2. www.wikipedia.org

3. Training Module for Interns, Idea Cellular.

http://www.ideacellular.com/

http://www.wikipedia.org/


Glossary

MS: Mobile Station

ME: Mobile Equipment

SIM: Subscriber Identity Module

PIN: Personal Identity Number

IMEI: International Mobile Equipment Identity Number

IMSI: International Mobile Subscriber Identity Number

TMSI: Temporary Mobile Subscriber Identity Number

LAI: Local Area Identity

ISDN: International Subscriber Digital Network

PSTN: Public Switched Telephone Network

PLMN: Public Land Mobile Network

MSC: Mobile Services Switching Network

HLR: Home Location Register

VLR: Visitor Location Register

EIR: Equipment Identity Register

DTI: Data Transfer Interface

EC: Echo Canceller

MC: Message Center

BSS: Base Station System

BSC: Base Station Controller

BTS: Base Transceiver Station

XCDR: Transcoder

AUC: Authentication Center

IWF: Interworking Function

EC: Echo Canceller


RAND: Randomly Generated Number

SRES: Signed Response

Ki: Authentication Key

Kc: Cipher Key (same as Ke, Encryption Key)

BCCH: Broadcast Control Channel

SDCCH: Stand-alone Dedicated Control Channel

MTC: Mobile Terminating Call

MOC: Mobile Originating Call

MMC: Mobile to Mobile Call

MSRN: Mobile Subscriber Roaming Number

LU: Location Update

LTG: Line/Trunk Groups

Final Report

Documents

Transcript of Final Report